High-strength aluminum alloy extruded shape exhibiting excellent corrosion resistance, ductility, and hardenability, and method for producing the same

ABSTRACT

An Al—Mg—Si-based high-strength aluminum alloy extruded shape exhibits excellent corrosion resistance and ductility, and exhibits excellent hardenability during extrusion (i.e., ensures high productivity). A method for producing the same is also disclosed. The high-strength aluminum alloy extruded shape includes 0.65 to 0.90 mass % of Mg, 0.60 to 0.90 mass % of Si, 0.20 to 0.40 mass % of Cu, 0.20 to 0.40 mass % of Fe, 0.10 to 0.20 mass % of Mn, and 0.005 to 0.1 mass % of Ti, with the balance being Al and unavoidable impurities, the aluminum alloy extruded shape having a stoichiometric Mg 2 Si content of 1.0 to 1.3 mass %, an excess Si content relative to stoichiometric Mg 2 Si of 0.10 to 0.30 mass %, and a total content of Fe and Mn of 0.35 mass % or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/JP2013/052002 filed on Jan. 30, 2013, and publishedin Japanese as WO 2013/115227 A1 on Aug. 8, 2013. This applicationclaims priority to Japanese Application No. 2012-018486 filed on Ja. 31,2012. The disclosures of the above applications are incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to an extruded shape produced using anAl—Mg—Si-based aluminum alloy.

BACKGROUND ART

In recent years, a reduction in weight of automobiles aimed to improvedriving performance and reduce fuel consumption has been desired fromthe viewpoint of environment protection.

Use of an aluminum alloy extruded shape as an automotive structuralmaterial has been studied in order to meet the requirements for reducingfuel consumption by way of a reduction in weight.

An automotive structural material is required to exhibit high strength,high bendability, and high corrosion resistance, and a JIS 7000 seriesaluminum alloy (Al—Zn—Mg-based aluminum alloy) and a JIS 6000 seriesaluminum alloy (Al—Mg—Si-based aluminum alloy) have attracted attention.However, a 7000 series aluminum alloy (natural age hardening alloy) hasa drawback in that processing becomes difficult due to hardening whenthe time elapsed from extrusion to bending is long. Moreover, a 7000series aluminum alloy shows a decrease in corrosion resistance under astress environment.

Therefore, a 6000 series aluminum alloy has been considered to be apromising heat-treatable alloy that does not undergo natural agehardening, and exhibits excellent corrosion resistance.

An extruded shape formed of a known high-strength 6000 series aluminumalloy exhibits high tensile strength, but exhibits insufficientelongation, and easily produces cracks during bending.

In order to obtain high strength, water-cooling press quenching isperformed immediately after extrusion.

The water-cooling press quenching treatment has an advantage in thatproperties similar to those obtained by solution/quenching treatmentthat reheats the extruded alloy after extrusion can be obtained.However, since a difference is in cooling rate occurs between eachcross-sectional area due to the cross-sectional shape of the extrudedshape, the difference in thickness, and the like, the extruded shapeshows a non-uniform temperature distribution during cooling, and strainoccurs. Therefore, the dimensional accuracy deteriorates, and it isdifficult to reduce the thickness of the cross-sectional profile. Thedegree of freedom of the cross-sectional shape decreases as a result ofpreventing occurrence of such strain.

The water-cooling press quenching treatment has another disadvantage inthat an increase in cost occurs as compared with an air-coolingquenching treatment.

On the other hand, the air-cooling quenching treatment has an advantagein that cost can be reduced as compared with the water-cooling pressquenching treatment. However, since the cooling rate is limited, highstrength may not be obtained depending on the alloy composition, and adeterioration in ductility may occur although high strength can beobtained.

JP-A-2002-285272 discloses an aluminum alloy extruded shape thatexhibits excellent axial crush properties and corrosion resistance, andincludes 0.4 to 0.8% of Mg, 0.3 to 0.9% of Si, 0.05% or less of Cu, and0.095% or less of Mn, Cr, Zr in total, wherein the number of Mg2Simoieties having a length of 3 pm in the extrusion direction is 50 ormore per mm². However, it is considered that the alloy compositiondisclosed in JP-A-2002-285272 provides excellent corrosion resistance,but achieves a proof stress of only about 220 MPa (i.e., cannotsufficiently contribute to a reduction in weight of the product). Sincea water-cooling press quenching treatment is normally used inJP-A-2002-285272, it is considered that the extrusion productivity islow.

Since Cu, Mn, Cr, and Zr are considered to be impurities, and thecontent thereof is limited, it is considered that an improvement inductility cannot be is achieved.

JP-A-2004-225124 discloses an aluminum alloy extruded shape thatexhibits excellent hardenability and axial crush properties, andincludes 0.45 to 0.75% of Mg, 0.45 to 0.80 of Si, 0.1 to 0.4% of excessSi, 0.15 to 0.40% of Mn, and 0 to 0.1% of Cr, wherein Mn and Crcompounds are finely dispersed. JP-A-2004-225124 achieves goodproductivity by utilizing an air-cooling press quenching treatment.However, the aluminum alloy extruded shape disclosed in JP-A-2004-225124has a proof stress of only about 220 MPa.

Since it is necessary to add Cr that achieves sharp quench sensitivity,it is difficult to improve the proof stress using an air-cooling means.

SUMMARY OF THE INVENTION Technical Problem

An object of the invention is to provide an Al—Mg—Si-based high-strengthaluminum alloy extruded shape that exhibits excellent corrosionresistance and ductility, and exhibits excellent hardenability duringextrusion (i.e., ensures high productivity), and a method for producingthe same.

Solution to Problem

According to one aspect of the invention, there is provided ahigh-strength aluminum alloy extruded shape that exhibits excellentcorrosion resistance, ductility, and hardenability, the aluminum alloyextruded shape including 0.65 to 0.90 mass % of Mg, 0.60 to 0.90 mass %of Si, 0.20 to 0.40 mass % of Cu, 0.20 to 0.40 mass % of Fe, 0.10 to0.20 mass % of Mn, and 0.005 to 0.1 mass % of Ti, with the balance beingAl and unavoidable impurities, the aluminum alloy extruded shape havinga stoichiometric Mg₂Si content of 1.0 to 1.3 mass %, an excess Sicontent relative to stoichiometric Mg₂Si of 0.10 to 0.30 mass %, and atotal content of Fe and Mn of 0.35 mass % or more. Note that the unit“mass %” may be hereinafter referred to as “%”.

The extruded shape is obtained by extruding an aluminum alloy having theabove composition, cooling the extruded aluminum alloy at an averagecooling rate of 100° C./min or less immediately after the extrusion, andsubjecting the cooled aluminum alloy to artificial aging.

When the average cooling rate is 100° C./min or less, it suffices toair-cool the aluminum alloy using a fan immediately after the extrusioninstead of water-cooling the aluminum alloy, and press quenching byair-cooling can be implemented.

For example, a cooling rate of 50 to 100° C./min can be achieved bycooling the extruded shape extruded from an extrusion press using a fan.

The extruded shape thus produced has a structure in which crystal grainshaving an aspect ratio of 4.0 or more have an average crystal grain sizeof 80 pm or less, and has a 0.2% proof stress (a) of 280 MPa or more.

The term “aspect ratio” used herein refers to the ratio (L₁/L₂) of thelength L₁ of the crystal grains of the recrystallized structure in theextrusion direction to the length L₂ of the crystal grains in thedirection orthogonal to the extrusion direction.

The term “average crystal grain size” used herein refers to the averagediameter of circles respectively circumscribed to the crystal grains.

The extruded shape according to one aspect of the invention has animpact strength determined by a Charpy impact test of 20 J/cm² or more.

The content range of each component is selected for the followingreasons.

Mg and Si

Mg and Si contribute to an improvement in the strength of the extrudedshape through formation of Mg₂Si precipitates.

Since a decrease in extrudability occurs if the Mg content and/or the Sicontent is too high, the upper limit of the Mg content is set to 0.90%,and the upper limit of the Si content is set to 0.90%.

The Mg₂Si content is set to 1.0 to 1.3% in order to obtain a 0.2% proofstress of 280 MPa or more while taking account of extrudability.

Excess Si relative to stoichiometric Mg₂Si can improve the 0.2% proofstress without significantly impairing extrudability.

However, a decrease in ductility may occur if the excess Si contentrelative to stoichiometric Mg₂Si is too high. Therefore, the excess Sicontent relative to stoichiometric Mg₂Si is set to 0.10 to 0.30%.

It is preferable to control the excess Si content relative tostoichiometric Mg₂Si within the range of 0.10 to 0.20% from theviewpoint of ensuring excellent ductility.

Cu

Cu contributes to solid solution hardening, and ensures elongation whenthe Cu content is within a given range.

Since a decrease in corrosion resistance and extrudability occurs if theCu content is too high, the Cu content is set to 0.2 to 0.4%.

Fe

One aspect of the invention is characterized in that the Fe content isset to 0.20 to 0.40%.

Fe refines the crystal grains of the extruded metal structure, andimproves ductility.

Known refinement components such as Mn, Cr, and Zr increase quenchsensitivity even during air-cooling using a fan immediately afterextrusion. In contrast, Fe does not increase quench sensitivity, andmakes it possible to perform quenching at a cooling rate of 100° C./minor less.

Mn

It is known that Mn affects quench sensitivity during air-cooling usinga fan immediately after extrusion. The inventor of the inventionconducted extensive studies, and found that Mn does not significantlyaffect quench sensitivity during air-cooling using a fan when the Mncontent is 0.20% or less. The inventor also found that, when the Mncontent is 0.10 to 0.20%, a recrystallized structure that extends in theextrusion direction is obtained in which propagation of cracks issuppressed as compared with a spherical recrystallized structure, andthe crystal grains have a small average crystal grain size.

Therefore, the total content of Fe and Mn is set to 0.35% or more.

Ti

Ti refines the crystal grains when casting a billet subjected toextrusion. The Ti content is preferably 0.005 to 0.10%.

If the Ti content exceeds 0.10%, coarse intermetallic compounds may beeasily produced, and may not disappear during extrusion. As a result,the strength of the extruded shape may decrease.

Additional Components

Additional components (e.g., Cr, Zr, and Zn) other than the abovecomponents may be included in the extruded shape as unavoidableimpurities as long as the content of each additional component is 0.05%or less, and the total content of additional components is 0.15% orless.

Advantageous Effects of the Invention

According to one aspect of the invention, the proof stress can be isimproved while ensuring extrudability by setting the stoichiometricMg₂Si content to 1.00 to 1.30%, and setting the excess Si contentrelative to stoichiometric Mg₂Si to 0.10 to 0.30%. It is possible toachieve high strength and high ductility by press quenching viaair-cooling in case that the Fe content is set to 0.20 to 0.40%, and theMn content is set to 0.10 to 0.20% so that “Fe+Mn≧0.35 mass %” issatisfied.

It is also possible to improve the impact strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the composition of each billet used for experiments andevaluations.

FIG. 2 shows the production conditions used for experiments andevaluations.

FIG. 3 shows evaluation results.

FIG. 4 shows an example of a comparison of the metal structure of anextruded shape.

DESCRIPTION OF EMBODIMENTS

Billets that differ in chemical composition were cast, extruded, andevaluated as described below.

A molten metal including the alloy components shown in FIG. 1 was isprepared, and cast at a casting speed 60 mm/min or more to obtain acylindrical billet having a diameter of 8 inches.

FIG. 2 shows the subsequent production conditions.

The cast billet was homogenized at 565 to 595° C. for 2 to 6 hours (see“HOMO conditions”).

The billet was preheated to 480 to 520° C., and extruded to obtain anextruded shape having a hollow cross-sectional shape (single-hollowcross-sectional shape) (W=50 mm, H=40 mm, t (thickness)=3 mm).

FIG. 2 shows the extrusion speed and the cooling rate.

The cooling rate was set to 50 to 100° C./min in order to achieve pressquenching by air-cooling using a fan. Note that the cooling rate was setto 200° C./min in Comparative Example 5.

The extruded shape was cooled to room temperature, and subjected toartificial aging at 185 to 200° C. for 3 to 3.5 hours (see “Heattreatment conditions”).

FIG. 3 shows the property evaluation results for the extruded shape thusproduced.

Evaluation items and Evaluation Methods(1) Tensile strength, 0.2% proof stress, and elongation: A JIS No. 4tensile test specimen was prepared from the extruded shape in accordancewith JIS Z 2241. The specimen was subjected to a tensile test using atensile tester compliant to the JIS standard.(2) Microstructure: A specimen was cut from the extruded shape,mirror-finished, and etched at 40° C. for 3 minutes using a 3% NaOHaqueous solution. The surface of the specimen was observed using anoptical microscope.

FIG. 4 shows a photograph of the metal structure of Comparative Example1 (see “RELATED-ART ALLOY”), and a photograph of the metal structure ofExample 1 (see “INVENTIVE ALLOY”).

The aspect ratio was determined by calculating the average value (n=5 to10) of the ratios (L₁/L₂) of the length L₁ of the crystal grains in theextrusion direction to the length L₂ of the crystal grains in thedirection orthogonal to the extrusion direction.

The crystal grain size was determined by calculating the average value(n=5 to 10) of the diameters of circles respectively circumscribed tothe crystal grains.

(3) Corrosion resistance: The stress corrosion cracking resistance (SCCresistance) was evaluated.

A No. 1 specimen was prepared in accordance with JIS H 8711, andsubjected to the following cycle test in a state in which a stress equalto 100% of the 0.2% proof stress was applied.

A cycle (3.5% NaCl aqueous solution, 25° C., 10 min→air-drying (25° C.,40% (humidity), 50 min)) is repeated 720 times, and a case where nocracks were observed was evaluated as acceptable.

(4) Impact strength: A JIS V-notch No. 4 tensile test specimen wasprepared from the extruded shape in accordance with JIS Z 2242. Theimpact strength was measured using a Charpy impact tester compliant tothe JIS standard.

The target impact strength was set to 20 J/cm² or more.

Evaluation Results

The extruded shapes of Examples 1 to 10 had a flat recrystallized metalstructure (microstructure) in which crystal grains having an aspectratio of 4.0 or more had an average crystal grain size of 80 μm or less.

The extruded shapes of Examples 1 to 10 had a proof stress of 280 MPa ormore (i.e., exhibited high strength), and had an elongation (ductility)of 8% or more.

The extruded shapes of Examples 1 to 10 had a Charpy impact strength of20 J/cm² or more.

The extruded shapes of Comparative Examples 1 to 5 showed highelongation, but had low proof stress.

The extruded shapes of Comparative Examples 1 to 3 had low proof stresssince the Cu content and the excess Si content were low.

The extruded shape of Comparative Example 4 had low proof stress sincethe Mg₂Si content was low, and the extruded shape of Comparative

Example 5 had low proof stress since the excess Si and the total contentof Mn and Fe were low.

The extruded shapes of Comparative Examples 6 to 8 are poor in both ofproof stress and elongation.

This is because the Fe content, the Cu content, and the Mg content werelow.

The extruded shapes of Comparative Examples 9 to 13 achieved the targetproof stress, but had low elongation and low impact strength.

This is because the total content of Fe and Mn was low. The extrudedshape of Comparative Example 14 had low proof stress, low elongation,and low impact strength since the excess Si content and the totalcontent of Fe and Mn were low.

The extruded shape of Comparative Example 15 had low proof stress sincethe excess Si content was low although the Si content and the Mg contentwere sufficient.

INDUSTRIAL APPLICABILITY

Since the aluminum alloy extruded shape according to the embodiments ofthe invention exhibits excellent corrosion resistance, ductility, andhardenability, the aluminum alloy extruded shape may be widely used asstructural materials for vehicles, machines, and the like.

1. A high-strength aluminum alloy extruded shape that exhibits excellentcorrosion resistance, ductility, and hardenability, the aluminum alloyextruded shape comprising 0.65 to 0.90 mass % of Mg, 0.60 to 0.90 mass %of Si, 0.20 to 0.40 mass % of Cu, 0.20 to 0.40 mass % of Fe, 0.10 to0.20 mass % of Mn, and 0.005 to 0.1 mass % of Ti, with the balance beingAl and unavoidable impurities, the aluminum alloy extruded shape havinga stoichiometric Mg₂Si content of 1.0 to 1.3 mass %, an excess Sicontent relative to stoichiometric Mg₂Si of 0.10 to 0.30 mass %, and atotal content of Fe and Mn of 0.35 mass % or more.
 2. The high-strengthaluminum alloy extruded shape as defined in claim 1, wherein crystalgrains of the aluminum alloy extruded shape having an aspect ratio of4.0 or more have an average crystal grain size of 80 μm or less.
 3. Thehigh-strength aluminum alloy extruded shape as defined in claim 1,wherein the aluminum alloy extruded shape has a proof stress of 280 MPaor more.
 4. The high-strength aluminum alloy extruded shape as definedin claim 1, wherein the aluminum alloy extruded shape has an impactstrength determined by a Charpy impact test of 20 J/cm² or more.
 5. Amethod for producing a high-strength aluminum alloy extruded shape thatexhibits excellent corrosion resistance, ductility, and hardenability,the method comprising extruding an aluminum alloy, cooling the extrudedaluminum alloy at an average cooling rate of 100° C./min or lessimmediately after the extrusion, and subjecting the cooled aluminumalloy to artificial aging, the aluminum alloy comprising 0.65 to 0.90mass % of Mg, 0.60 to 0.90 mass % of Si, 0.20 to 0.40 mass % of Cu, 0.20to 0.40 mass % of Fe, 0.10 to 0.20 mass % of Mn, and 0.005 to 0.1 mass %of Ti, with the balance being Al and unavoidable impurities, thealuminum alloy having a stoichiometric Mg₂Si content of 1.0 to 1.3 mass%, an excess Si content relative to stoichiometric Mg₂Si of 0.10 to 0.30mass %, and a total content of Fe and Mn of 0.35 mass % or more.
 6. Thehigh-strength aluminum alloy extruded shape as defined in claim 2,wherein the aluminum alloy extruded shape has a proof stress of 280 MPaor more.
 7. The high-strength aluminum alloy extruded shape as definedin claim 2, wherein the aluminum alloy extruded shape has an impactstrength determined by a Charpy impact test of 20 J/cm² or more.